Electronic devices comprising active organic materials are attracting increasing attention for use in devices such as organic light emitting diodes, organic photovoltaic devices, organic photosensors, organic transistors and memory array devices. Devices comprising organic materials offer benefits such as low weight, low power consumption and flexibility. Moreover, use of soluble organic materials allows use of solution processing in device manufacture, for example inkjet printing or spin-coating.
A typical organic light-emissive device (“OLED”) is fabricated on a glass or plastic substrate coated with a transparent anode such as indium-tin-oxide (“ITO”). A layer of a thin film of at least one electroluminescent organic material is provided over the first electrode. Finally, a cathode is provided over the layer of electroluminescent organic material. Charge transporting, charge injecting or charge blocking layers may be provided between the anode and the electroluminescent layer and/or between the cathode and the electroluminescent layer.
In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the organic electroluminescent layer to form an excitons which then undergo radiative decay to give light.
In WO90/13148 the organic light-emissive material is a conjugated polymer such as poly(phenylenevinylene). In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as tris-(8-hydroxyquinoline) aluminium (“Alq3”). These materials electroluminesce by radiative decay of singlet excitons (fluorescence) however spin statistics dictate that up to 75% of excitons are triplet excitons which undergo non-radiative decay, i.e. quantum efficiency may be as low as 25% for fluorescent OLEDs-see, for example, Chem. Phys. Lett., 1993, 210, 61, Nature (London), 2001, 409, 494, Synth. Met., 2002, 125, 55 and references therein.
It has been postulated that the presence of triplet excitons, which may have relatively long-lived triplet excited states, can be detrimental to OLED lifetime as a result of triplet-triplet or triplet-singlet interactions (“lifetime” as used herein in the context of OLED lifetime means the length of time taken for the luminance of the OLED at constant current to fall by 50% from an initial luminance value, and “lifetime” as used herein in the context of lifetime of a triplet excited state means the half-life of a triplet exciton). US 2007/145886 discloses an OLED comprising a triplet-quenching material to prevent or reduce triplet-triplet or triplet-singlet interactions between molecules in the excited state by quenching undesirable triplet excited states without quenching the excited singlet states that decay by fluorescent emission.
WO 2005/043640 discloses that blending a perylene derivative with an organic light-emissive material in an organic light-emissive device can give a small increase in the lifetime of the device. However, while higher concentrations of perylene derivative give greater improvements in lifetime this results in a significant red-shift in the emission spectrum.
US 2007/145886 discloses an OLED comprising a triplet-quenching material to prevent or reduce triplet-triplet or triplet-singlet interactions.
US 2005/095456 discloses an OLED having a light-emitting layer comprising a host material, a dye or pigment and an additive exhibiting an absorption edge of which energy level is higher than that of an absorption edge of the dye or the pigment
However, quenching of triplet excitons as described in US 200/145886 inevitably means that the energy of the triplet excitons is lost in non-light emitting pathways (such as heat).